Electrodeionization apparatuses such as the electrodeionization apparatus 1 shown in FIG. 4 are conventionally used for producing deionized water that is used in, for instance, semiconductor manufacturing plants, liquid crystal manufacturing plants, and in various industries such as the pharmaceutical, foodstuff and power industries, as well as in consumer and research facilities (Patent documents 1 to 3). Such an electrodeionization apparatus 1 comprises concentrating chambers 15 and desalination chambers 16 formed alternately by alternately arranging a plurality of anion exchange membranes (A membranes) 13 and cation exchange membranes (C membranes) 14 between electrodes (an anode 11, a cathode 12, an anode chamber 17 and a cathode chamber 18). The desalination chambers 16 are filled with an anion exchanger and a cation exchanger comprising, for instance, an ion exchange resin, ion exchange fibers or a graft exchanger, in a mixed or multilayered state.
Water dissociation in the electrodeionization apparatus gives rise to H+ ions and OH− ions that regenerate continuously the ion exchangers that fill the desalination chambers, thereby enabling an efficient desalination treatment. Herein there is required no regeneration process using chemicals, as is the case in conventional ion exchangers that have been widely used heretofore. Such electrodeionization apparatuses elicit thus the superior effect of providing high-purity water that can be obtained in a completely continuous manner.
When using directly tap water, obtained by subjecting river water, groundwater or the like to clarification, dechlorination and softening treatments in a water treatment plant, as the water to be treated in electrodeionization apparatuses, or when the water to be treated has a high calcium concentration, (1) scale forms in the concentrating chambers, and (2) the conductivity of the treated water deteriorates on account of increased CO2 load. Therefore, such waters are not processed directly as water to be treated in electrodeionization apparatuses.
Among the above problems, the increased CO2 load (2) can be solved by using a comparatively inexpensive decarbonation apparatus as a pre-treatment apparatus of the electrodeionization apparatus. To prevent formation of scale in the concentrating chambers, as in (1), there are methods that involve removing completely hard components from the water to be treated, by providing a softener or the like before the electrodeionization apparatus. A softener, however, has to be regenerated. This nullifies the advantages of using an electrodeionization apparatus, which does not require regeneration.
To solve the above problem, known methods involve disposing a reverse osmosis membrane device (RO membrane device) as a pre-treatment apparatus of the electrodeionization apparatus, with a view to reducing the concentration of hard components and CO2. An ordinarily employed method resorts to arranging two RO membrane devices in series, in particular when the hard component concentration in the water to be treated is high.
Methods have been proposed (Patent documents 4 and 5) in which the concentrating chambers 15 of a electrodeionization apparatus 10 are partitioned by bipolar membranes 20, as shown in FIG. 4, to prevent thereby calcium ions (Ca2+) and carbonate ions (CO32−) from meeting and giving rise to a scale component in the concentrating chamber 15. This allows omitting the RO membrane device that had been necessary as a pre-treatment apparatus of the electrodeionization apparatus 10. Equipment costs and process costs can be reduced accordingly.    Patent document 1: Japanese Patent No. 1782943    Patent document 2: Japanese Patent No. 2751090    Patent document 3: Japanese Patent No. 2699256    Patent document 4: Japanese Patent Application Laid-open No. 2001-198577    Patent document 5: Japanese Patent Application Laid-open No. 2002-186973